1. Field of the Invention
The present invention relates to the refining of hydrocarbon feedstocks. More particularly, this invention concerns the segregation and removal of coke precursors from atmospheric and vacuum residuum having a Conradson carbon residue of at least about 10 wt.%.
2. Description of Relevant Art
Hydrocarbon feedstocks, whether derived from natural petroleum or synthetic sources, are composed of hydrocarbon and non-hydrocarbon (e.g. heteroatom containing organic molecules) components which differ in boiling point, molecular weight and chemical structure. High boiling, high molecular weight non-hydrocarbons (e.g. asphaltenes) are known to contain a greater proportion of carbon forming constituents (i.e. coke precursors) than lower boiling naphtha and distillate fractions. Because coke precursors form coke during thermal processing (such as is employed in a modern refinery), it is desirable to remove (or at least segregate) the non-hydrocarbon components containing the coke precursors, thereby facilitating further processing of the more valuable fractions of the feedstock. Two methods often utilized to segregate are distillation and solvent deasphalting.
Distillation physically separates a hydrocarbon feedstock into contiguous fractions, each of which is characterized by a specific boiling range and molecular weight. While distillation can effectively reject carbon forming constituents, it has been found that a significant portion of the nonvolatile residue contains valuable hydrocarbons low in coke precursors but too high in molecular weight to distill. Such results are particularly noticeable with heavy hydrocarbon feedstocks such as heavy crudes and oils.
Deasphalting is a solvent extraction process utilizing a light hydrocarbon solvent (e.g., propane, butane or heptane) to separate heavy hydrocarbon feedstocks into a deasphalted oil and a low value residue or asphalt which contains asphaltenes. Unfortunately, the separation is not selective in that much of the more valuable deasphalted oil is precipitated with the residue while hydrocarbons containing coke precursors are extracted with the deasphalted oil.
Thus, both distillation and deasphalting, while upgrading hydrocarbon feedstocks by separation into high and lower boiling fractions, only partially segregate the coke precursors from the more valuable fractions. More importantly, with each process, a significant portion of the more valuable product inherently and unavoidably remains with the coke precursor rich residue. This is particularly so with heavy crudes and oils. Therefore, it would be desirable to have available a simple and convenient method which selectively removes coke precursors from a feedstock and minimizes the loss of more valuable hydrocarbons inherent in conventional separation processes.
Solvent extractions and various other techniques have been proposed for preparation of Fluid Catalytic Cracking (FCC) charge stock from resids. Solvent extraction, in common with propane deasphalting, functions by selection on chemical type, rejecting from the charge stock the aromatic compounds which can crack to yield octane components of cracked naphtha. Low temperature, liquid phase sorption on catalytically inert silica is described by Shuman et al, Oil and Gas Journal, Apr. 6, 1953, page 113. U.S. Pat. Nos. 3,565,795 and 3,567,627 describe a method of separating polar materials from petroleum distillate fractions by selective solvent extraction.
U.S. Pat. No. 2,472,723 describes a catalytic cracking process whereby an adsorptive clay is added to the charge to adsorb the polynuclear aromatic compounds which are believed to be coke precursors and thus reduce the amount of coke deposited on the active cracking catalyst. This process suffers, however, in that the adsorptive clay containing the polar molecules is fed through the cracking zone and regenerator of the cracking apparatus and must then be separated from the active cracking catalyst, which has significantly higher catalytic activity than the clay.